The signing fingerprint you specified is already used by another Android OAuth2 client
Some time ago I created an example project (lets call it "example project") with Oauth2 client id for android application in Google APIs console. I also added SHA1 fingerprint and package name (for example com.package.name).
My mistake was that an application with same package name already existed. Now I need to create an Oauth2 client id for android application in the "valid project" with package name com.package.name and with SHA1 fingerprint which I added before. Obviously when I tried to add this fingerprint I got an error.
The signing fingerprint you specified is already used by another Android OAuth2 client.
After that I remembered about "example project" and deleted client id from this project. The problem is that I still not able to add this fingerprint for package name com.package.name. Client id is deleted but still I have the same error as above.
So do I have a possibility to use this fingerprint for the same package name in another project?
Answer
This error occurs when there is an existing SIGNING CERTIFICATE FINGERPRINT (SHA1) on Google Developers linked to a different account.
Solution is to delete the project that is using that SHA1 in console.developers.google.com for the old/incorrect project.
You cannot change the SHA1 fingerprint once it is set. Remember that deleting the project takes 7 days to completely remove it.
Another option is to delete the debug.keystore and generate a new one with:
keytool -genkey -v -keystore debug.keystore -alias androiddebugkey
-storepass android -keypass android -keyalg RSA -validity 14000
Remember you have to uninstall the app otherwise you get the [INSTALL_FAILED_UPDATE_INCOMPATIBLE] error.